alignment of popup box

I created a popup box at the welcome page with this website:
html-online.com/articles/simple-popup-box/
When you click on "Kontakt", the popup box opens but it is not aligned in the center, see screenshot.
Can someone please help me with the background of the popup box? It would be fine if the background would go over the whole screen (by staying responsive for mobile devices). Or the background would be centered. I guess it is a something with css?
This is the css class that is responsible for the background of the popup box:

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There are some difference in bootsrap 3 and bootsrap 4 , better to put the link to bootsrap 3 for LimeSurvey

Assistance on LimeSurvey forum and LimeSurvey core development are on my free time.
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professional service on demand
(or search sondages pro).
An error happen ? Before make a new topic : remind the
Debug mode
.

There ist just a last positioning thing on my welcome page: I tried to move the button down to the same height of the "next"/"start" button. Is it not allowed to use the classes without the "#question{QID}" in javascript?

Assistance on LimeSurvey forum and LimeSurvey core development are on my free time.
I'm not a LimeSurvey GmbH member,
professional service on demand
(or search sondages pro).
An error happen ? Before make a new topic : remind the
Debug mode
.

I discovered that when the modal-button is opened on a small screen width, the modal-dialog is shifted down to the bottom of the page, outside of the screen. I also tried to center the modal-dialog with
getbootstrap.com/docs/4.0/components/modal/#vertically-centered
but it has no effect. I think this problem has to do with my custumized theme, because there isn't this problem with the standard shipped themes.
When I change in theme options "bootswatch theme" to another theme than spacelab, it works. But I would like to use my current bootswatch theme "spacelab".

Do you have any idea how to vertical align the modal-dialog to the top?